Coordinate-capturing apparatus

ABSTRACT

A coordinate-capturing apparatus suitable for inputting handwritten characters or diagrams to a computer or a printer is disclosed. The apparatus includes an ultrasonic transmitter mounted on an input pen for transmitting ultrasonic waves, two ultrasonic receivers for receiving ultrasonic waves transmitted form the ultrasonic transmitter, and a distance calculating device for calculating distances between the input pen and the two ultrasonic receivers in order to determine coordinates representing a position of the input pen on the basis of the calculated distances. The apparatus of the invention can perform coordinate detection smoothly even when the input pen moves fast, and has reduced electric power consumption in the input pen to increase the life-span of a battery included in the input pen.

FIELD OF THE INVENTION

[0001] The present invention relates to a coordinate-capturing apparatus, to be more specific, relates to a coordinate-capturing apparatus suitable for inputting handwritten characters or diagrams to a computer or a printer.

BACKGROUND OF THE INVENTION

[0002] There are various types of coordinate-capturing system using a tablet and an input pen, including a pressure-sensitive type, an electrostatic-coupling type, an electromagnetic-coupling type, an ultrasonic in-medium propagation (surface acoustic wave) type, and an ultrasonic aerial-propagation type.

[0003] In each of the above conventional types except the ultrasonic aerial-propagation type, a sensor is built into a medium in the shape of a plate. Accordingly, they become too cumbersome to carry since their dimensions are determined by their input surfaces.

[0004] On the other hand, the ultrasonic aerial-propagation type has not the constraint of the input surface, and any surface can be used as the input surface. Japanese Unexamined Utility Model Publication No. 64-7340 discloses determining a position of a sounding object by receiving a sound wave emitted from the object by at least three microphones disposed on the same plane, determining at least two hyperbolic curves on each of which two associated microphones lie, on the basis of difference in timing of reception of the sound wave between the two associated microphones, and determining a point of intersection of at least two determined hyperbolic curves (refereed to as “3-ultrasonic-wave type”). U.S. Pat. No. 4,814,552 discloses a different method for determining coordinates of a position of a pen. In this method, an ultrasonic signal for position sampling and an electromagnetic signal for timing detection of emission of the ultrasonic signals are emitted from the pen, and are received by two ultrasonic receivers and one electromagnetic signal receiving element respectively. Then, time delays between reception of the electromagnetic signal and receptions of the ultrasonic sampling signal by the two ultrasonic receivers are measured to calculate distances between the pen and the ultrasonic receivers taking account the propagation speed of the ultrasonic signal, thereby determining the position of the input pen by trigonometry (referred to as “2-ultrasonic-wave+electromagnetic-wave type”).

OBJECTS AND SUMMARY OF THE INVENTION

[0005] The above 3-ultrasonic-wave type has a problem that the determination of the position of the sounding object is greatly affected by the accuracy of positioning the three microphones, and therefore it is difficult to carry out the position-determination, with high precision and high accuracy. In addition, since it is unknown at which microphone the ultrasonic signal arrives first, it is necessary to watch for the ultrasonic signal by the three microphones continuously, which requires a complicated circuit structure to perform complicated calculations, and therefore increases the cost of manufacturing.

[0006] In the 2-ultrasonic-wave+electromagnetic-wave type, to obtain a smooth curve of detected coordinates even when the pen moves fast, it is necessary to shorten a sampling period, that is, to shorten the period of emission of the electromagnetic signal from the pen. Besides, since electric power drawn for emitting ultrasonic signals is large, power consumption in the input pen is large and therefore a battery included in the pen is short-lived.

[0007] An object of the present invention is to make it possible to perform coordinate detection smoothly even when the input pen moves fast.

[0008] Another object of the present invention is to reduce electric power consumption in the input pen to increase the life-span of a battery included in the input pen.

[0009] The above objects are achieved by a coordinate-capturing apparatus comprising:

[0010] an ultrasonic transmitter mounted on an input pen to be in contact with a medium for transmitting ultrasonic waves;

[0011] a first and a second ultrasonic receivers for receiving ultrasonic waves transmitted form said ultrasonic transmitter;

[0012] a distance calculating device for calculating distances between said input pen and said first and said second ultrasonic receivers when a specific condition is satisfied;

[0013] a coordinate calculating device for calculating coordinates representing a position of said input pen on the basis of said distances calculated by said distance calculating device; and

[0014] a distance variation calculating device for calculating variations of said distances between said input pen and said first and second ultrasonic receivers on the basis of time periods between receptions of an ultrasonic wave transmitted from said ultrasonic transmitter at a first time by said first and second ultrasonic receivers respectively and receptions of an ultrasonic wave transmitted from said ultrasonic transmitter at a second time by said first and second ultrasonic receivers respectively;

[0015] said distance calculating device calculating, on the basis of said distances calculated when said specific condition is satisfied and said variations of said distances calculated by said distance variation calculating device, distances between said input pen and said first and second ultrasonic receivers when said specific condition is not satisfied.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Embodiments of the invention will now be described by way of example and with reference to the accompanying drawings in which:

[0017]FIG. 1 is a schematically perspective view of a first embodiment;

[0018]FIG. 2 is a top view of a coordinate-capturing apparatus of the first embodiment;

[0019]FIG. 3 is a view showing coordinates to be captured by the coordinate-capturing apparatus of the first embodiment;

[0020]FIG. 4 is a block diagram showing a circuit structure of the coordinate-capturing apparatus of the first embodiment;

[0021]FIG. 5 is a time chart of various signals within the coordinate-capturing apparatus of the first embodiment;

[0022]FIG. 6 is a block diagram explaining a circuit structure of a coordinate-capturing apparatus of a second embodiment;

[0023]FIG. 7 is a block diagram explaining a circuit structure of a coordinate-capturing apparatus of a third embodiment;

[0024]FIG. 8 is a block diagram showing a circuit structure of a coordinate-capturing apparatus of a fourth embodiment;

[0025]FIG. 9 is a view showing coordinates to be captured by the coordinate-capturing apparatus of the fourth embodiment;

[0026]FIG. 10 is a time chart of various signals within the coordinate-capturing apparatus of the fourth embodiment;

[0027]FIG. 11 is a block diagram explaining a circuit structure of a coordinate-capturing apparatus of a fifth embodiment;

[0028]FIG. 12 is a block diagram explaining a circuit structure of a coordinate-capturing apparatus of a sixth embodiment;

[0029]FIG. 13 is a time chart of various signals within the coordinate-capturing apparatus of the sixth embodiment;

[0030]FIG. 14 is a block diagram explaining a circuit structure of a coordinate-capturing apparatus of a seventh embodiment;

[0031]FIG. 15 is a time chart of various signals within the coordinate-capturing apparatus of the seventh embodiment;

[0032]FIG. 16 is a view explaining a process for determining an input position in a coordinate-capturing apparatus of a ninth embodiment;

[0033]FIG. 17 is a top view of a coordinate-capturing apparatus of a tenth embodiment; and

[0034]FIG. 18 is a block diagram explaining a circuit structure of the coordinate-capturing apparatus of the tenth embodiment.

DESCRIPTION OF THE PREFERRED EMBODUMENTS

[0035]FIG. 1 is a schematically perspective view of an embodiment of the coordinate-capturing apparatus according to the invention, FIG. 2 is a top view of the apparatus, FIG. 3 is a view showing coordinates to be captured, FIG. 4 is a block diagram showing a circuit structure of the apparatus, and FIG. 5 is a time chart of various signals within the apparatus.

[0036] As shown in FIGS. 1 and 2, the coordinate-capturing apparatus includes an input pen 1 for handwriting characters into a medium 20 such as a paper form, a main unit 11 which detects the coordinates of a locus of a tip portion of the input pen 1 when characters are written into the medium 20 by the input pen. The input pen 1 is sticklike.

[0037] The main unit 11 is in the shape of a rectangular parallelepiped as a whole. The medium 20 and the main unit 11 are placed on a not illustrated plane, for example on a desktop. The medium 20 is placed on the plane such that its top side is in contact with or close to a side of the main unit 11.

[0038] The main unit 11 captures coordinates of a position of the input pen 1 between the moment at which the input pen 1 comes in contact with the medium 20 and the moment at which the input pen 1 separates from the medium 20 at intervals of a predetermined sampling period. A state in which the input pen 1 is in contact with the medium 20 is referred to as “pen-down state”, and a state in which the input pen 1 is separate from the medium 20 is referred to as “pen-up state” hereinafter. Furthermore, one action between a pen-down and a subsequent pen-up is referred to as one “pen-stroke” or “stroke”.

[0039] Characters written by the input pen 1 include not only alphanumeric characters, Chinese characters, and Japanese syllabaries but also symbols, diagrams, etc.

[0040] The input pen 1 has a writing member 2 such as a core of a ballpoint pen for inking characters into the medium 20. The writing member is not limited to a core of a ballpoint pen. Any writing member can be used if it is long-life and easy to replace. The writing member is not indispensable for the apparatus of this embodiment. A sheet-type ultrasonic oscillator 3 of a polymeric material is wounded onto the surface of a jacket 1 a at the tip portion of the input pen 1 (in the vicinity of the writing member 2). The ultrasonic oscillator 3 is not limited to such a sheet type of a polymeric material. For example, a piezoelectric device such as PZT (PbZrO₃—PbTiO₃) can be used. An LED (light emitting diode) 4 is provided in the vicinity of the ultrasonic oscillator 3.

[0041] A drive circuit 5 for driving the ultrasonic oscillator 3 and the LED 4, and a battery 6 are included in the input pen 1 (FIG. 2). The input pen 1 has a not illustrated switch for detecting a state in which the tip of the writing member 20 is pressed against the medium 20, that is, the pen-down state. The drive circuit 5 drives the ultrasonic oscillator 3 and the LED4 while the pen-down state is detected by this switch.

[0042] A cylindrical-shaped member 12 a is provided at one end portion of the main unit 11 so as to project vertically therefrom, and a sheet-type ultrasonic receiver 13 a of a polymeric material is wound onto the side of this member 12 a. A cylindrical-shaped member 12 b is provided at the other end portion of the main unit 11 so as to project vertically therefrom, and a sheet-type ultrasonic receiver 13 b of a polymeric material is wound onto the side of this member 12 b.

[0043] The ultrasonic receivers 13 a, 13 b are not limited to such a sheet-type of a polymeric material. Any piezoelectric device such as PZT (PbZrO3—PbTiO3) can be used. A light-receiving element 14 is provided between the ultrasonic receivers 13 a and 13 b. As shown in FIG. 3, the ultrasonic receivers 13 a, 13 b are located at the points R and S respectively on the straight line P, and separated from each other by the distance K. The medium 20 has a plane including the straight line P. The main unit 11 includes a signal processor 15 to which the ultrasonic receivers 13 a, 13 b, and the light-receiving element 14 are connected.

[0044] The ultrasonic oscillator 3 and the drive circuit 5 of the pen 1 form an ultrasonic transmitter. The ultrasonic transmitter of this embodiment transmits a wave group (an ultrasonic pulse train) including p pulses having a frequency f_(S) (p being 2 in FIG. 5) at a frequency f_(A)=f_(S)/h in the pen-down state.

[0045] The LED 4 and the drive circuit 5 form a light signal transmitter. The light signal transmitter of this embodiment transmits a wave group (a light pulse train) including q pulses having a frequency f_(S) (q being 2 in FIG. 5) at a frequency f_(B)=f_(A)/j (j being a positive integer greater than 2, for example, 10) in the pen-down state.

[0046] The signal processor 15 has a clock pulse generating circuit 22, a light-receiving circuit 24, ultrasonic-receiving circuits 25 a, 25 b, counter circuits 26 a, 26 b, 28 a, 28 b, 30 a, 30 b, registers 27 a, 27 b, 29 a, 29 b, 32 a, 32 b, a fixed register 21, a coordinate-determination part 23, a variation-calculating circuits 31 a, 31 b, and adders 53 a and 53 b.

[0047] The clock pulse generating circuit 22 generates clock pulses having a predetermined frequency F_(D) (3 MHz, for example) continuously at least in the pen-down state. The light-receiving circuit 24 receives an output of the light-receiving element 14 and outputs a corresponding pulse signal (FIG. 5(E)). This signal is used as a start signal S. The ultrasonic-receiving circuits 25 a, 25 b receive outputs of the ultrasonic receivers 13 a, 13 b, and output corresponding ultrasonic-reception pulse signals G_(R), G_(L) (FIG. 5(B) and (C)).

[0048] The counter circuits 26 a, 26 b receive outputs of the clock pulse generating circuit 22, the light-receiving circuit 24, and the ultrasonic-receiving circuit 25 a, 25 b to measure the times elapsed between the moment at which the ultrasonic signal is emitted with the start signal S form the ultrasonic oscillator 3 and the moment at which it is received by the ultrasonic receivers 13 a, 13 b.

[0049] The registers 27 a, 27 b hold the count values of the counter circuits 26 a, 26 b after completion of the counts (time clocking). Thereafter, the counter circuits 26 a and 26 b are reset to provide for the next counts.

[0050] The counter circuits 28 a, 28 b receive outputs of the pulse generating circuit 22 and the ultrasonic-receiving circuits 25 a, 25 b, and count the number of the clock pulses generated between two successive pulses included in a pulse train generated at the ultrasonic-receiving circuits 25 a, 25 b. Thus, the time interval or pulse interval between two successive pulses can be determined.

[0051] The registers 29 a, 29 b hold the count values of the counter circuits 28 a, 28 b after completion of the counts (time clocking). Thereafter, the counter circuits 28 a, 28 b are reset to provide for the next counts.

[0052] The counter circuits 30 a, 30 b receive outputs of the pulse generating circuit 22 and the ultrasonic-receiving circuits 25 a, 25 b, and count the number of the clock pulses between generation of one pulse train and generation of the succeeding pulse train at the ultrasonic-receiving circuits 25 a, 25 b, that is, between reception of one pulse train and reception of the succeeding pulse train at the ultrasonic receivers 13 a, 13 b. Thus, the time interval between two successive pulse trains generated at the ultrasonic-receiving circuits 25 a, 25 b, that is, the time interval between two successive pulse trains received at the ultrasonic receivers 13 a, 13 b can be determined.

[0053] The variation-calculating circuits 31 a, 31 b have multipliers 51 a, 51 b and subtracters 52 a, 52 b. The multipliers 51 a, 51 b multiply the output of the registers 29 a, 29 b by a constant h respectively. The outputs of the multipliers 51 a, 51 b represent a time interval between two successive ultrasonic pulse trains transmitted from the ultrasonic oscillator 3. The subtracters 52 a, 52 b subtract the outputs of the multipliers 51 a, 51 b from the outputs of the counter circuits 30 a, 30 b respectively. The outputs of the subtracters 52 a, 52 b, which are also the outputs of the variation-calculating circuits 31 a, 31 b, represent variations of the distances between the input pen 10 and the ultrasonic receivers 13 a, 13 b.

[0054] The registers 32 a, 32 b hold the outputs (distance variations) of the variation-calculating circuits 31 a, 31 b respectively.

[0055] The adders 53 a, 53 b add the values held in the registers 32 a, 32 b to the values held in the registers 27 a, 27 b respectively. The results of the additions are held in the registers 27 a, 27 b respectively.

[0056] The fixed register 21 holds a value (N_(K)) representing the distance between the ultrasonic receivers 13 a, 13 b. The coordinate-determination part 23 calculates coordinates (x-y coordinates, for example) representing a position of the input pen by trigonometry from the value held in the fixed register 21 and the values held in the registers 27 a, 27 b representing the distances between the input pen and the ultrasonic receivers 13 a, 13 b. The result of this calculation is transferred to an external unit (not shown) such as a memory or a computer, and stored there.

[0057] This embodiment further includes a controller 16 for controlling the above-described components.

[0058] The operation of this embodiment will now be explained referring to FIGS. 4, 5, etc. As a first step, the receiver-to-receiver distance N_(K) (the number of the clock pulses) representing the distance K between the ultrasonic receivers is calculated as described below and stored in the fixed register 21. When the propagation speed of an ultrasonic wave in the air is v and the time required for the ultrasonic wave to travel the distance K is t_(k), the equation K=v·t_(K) holds. When the frequency of the clock pulses generated by the clock pulse generating circuit 22 is F_(D), t_(K) can be represented by the number of the clock pulses as t_(K)=N_(K)/F_(D), and accordingly, the equation N_(K)=K·F_(D)/v . . . (A1) holds. The coordinate-determining part 23 of the signal processor 15 sets up a rectangular coordinate system using the above NK calculated by the equation (A1), in which the straight line P makes an x axis in an x-y plane, the point R makes an origin point R(0, 0), and the coordinates of the point S are S (N_(K), 0)

[0059] Here, F_(D)=3 MHz, K=23.1 cm, and the NK is calculated at 2100 from the equation (A1) assuming that the ultrasonic propagation speed is 330 m/sec. This calculated N_(K) is held in the fixed register 21.

[0060] The principal of coordinate-detection with the input pen 10 will be explained below. At the start of a pen stroke, when the writing member 2 of the input pen 1 comes into contact with the medium 20 at the point Q1 shown in FIG. 3, the not illustrated switch turns on the drive circuit 5, whereby the ultrasonic oscillator 3 emits an ultrasonic signal U_(S) and the LED 4 emits a light-signal E_(S) at the same time. This moment conforms to PEN-DOWN in FIG. 5. As shown in FIG. 5(A), the ultrasonic signal U_(S) is a wave group emitted at the frequency of f_(A)=f_(S)/h from the start of the pen-down or the pen stroke, each group ^(n)g^(m) (m=1,2, . . . ,j) including p pulses of the frequency of f_(S). In this embodiment, p=2. The ultrasonic signal U_(S) is received by the ultrasonic receivers 13 a, 13 b, and converted into electric pulses in the ultrasonic-receiving circuits 25 a, 25 b respectively to generate ultrasonic-reception signals G_(R) and G_(L). As shown in FIG. 5(D), the light signal E_(S) is a wave group emitted at the frequency of f_(B)=f_(A)/j (j being an integer equal to or greater than 2) from the start of the pen-down or the pen stroke, each group ^(n)e including q pulses of the frequency f_(S). In this embodiment, q=2. The light signal E_(S) is received by the light-receiving element 14, and processed in the light-receiving circuit 24 to generate the start signal S shown in FIG. 5(E). The time periods ¹T¹ _(R), ¹T¹ _(L) between generation of a first start signal ¹S and generation of first ultrasonic-reception signals ¹G¹ _(R) , ¹G¹ _(L) in response to a first wave group ¹g¹ of the ultrasonic signal U_(S) can be assumed to be propagation times required for the ultrasonic signal to travel from the ultrasonic oscillator 3 to the ultrasonic receivers 13 a, 13 b. The propagation times ¹T¹ _(R) and ¹T¹ _(L) are measured as the number of the clock pulses ¹N¹ _(R) and ¹N¹ _(L) having the frequency F_(D)=3 MHz generated by the clock pulse generating circuit 22, and stored temporarily in the registers 27 a and 27 b respectively. The numbers ¹N¹ _(R) and ¹N¹ _(L) of the clock pulses translate into the distances ¹L¹ _(R), ¹L¹ _(L) between the point Q₁, and the ultrasonic receivers 13 a, 13 b respectively. It means that they are expressed as the same unit as the receiver-to-receiver distance N_(K). As seen from FIG. 3, three line segments corresponding to the distances ¹N¹ _(R), ¹N¹ _(L), and N_(K) respectively form a triangle. Accordingly, the coordinate-determination part 23 can determine the position of the point Q₁ as its coordinates Q₁ (x₁, y₁) in the x-y coordinate system from the following equations (A2), (A3) by trigonometry. The determined position (coordinates) is stored in the memory (not shown), or transferred to the external unit such as a computer (not shown).

[¹ N ¹ _(R)]² =[x ₁]² +[y ₁]²  (A2)

[¹ N ¹ _(L)]² =[N _(K) −x ₁]² +[y ₁]²  (A3)

[0061] For example, if F_(D)=3 MHz, and it has been detected that ¹N¹ _(R)=1000, and ¹N¹ _(L)=1700, it is determined that x₁=600, and y₁=800 since N_(K)=2100. These determined values are stored in the memory (not shown), or transferred to the external unit such as a computer (not shown).

[0062] As described above, the distances between the pen and the ultrasonic receivers 13 a, 13 b are determined from the times elapsed between the emission of the ultrasonic signal along with the light signal and the receptions of the ultrasonic signal by the ultrasonic receivers 13 a, 13 b.

[0063] Subsequently, the counter circuits 28 a, 28 b count the numbers ¹Nc¹ _(R), ¹Nc¹ _(L) of the clock pulses generated at the frequency F_(D) by the clock pulse generating circuit 22 to measure periods ¹Tc¹ _(R), ¹Tc¹ _(L) of the pulses of the ultrasonic-reception signals ¹G¹ _(R), ¹G¹ _(L) having the frequency f_(S). These measurements are stored in the registers 29 a, 29 b and the counter circuits 28 a, 28 b are reset. Likewise, the counter circuits 30 a, 30 b count the numbers ¹Ns¹ _(R) and 1Ns1L of the clock pulses generated at the frequency F_(D) by the clock pulse generating circuit 22 to measure elapsed times ¹Ts¹ _(R), ¹Ts¹ _(L) between rises of the ultrasonic-reception signals ¹G¹ _(R), ¹G¹ _(L) and rises of second ultrasonic-reception signals ¹G² _(R), ¹G² _(L). At each of the risings of the second ultrasonic-reception signals ¹G² _(R), ¹G² _(L), the variation-calculating circuits 31 a, 31 b calculate the next variations ¹D² _(R), ¹D² _(L) by the following equations.

¹ D ² _(R)=¹ Ns ¹ _(R) −h· ¹ Nc ¹ _(R)  (A4)

¹ D ² _(L)=¹ Ns ¹ _(L) −h· ¹ Nc ¹ _(L)  (A5)

[0064] After the variations ¹D² _(R) and ¹D² _(L) are stored in the registers 32 a and 32 b, the registers 30 a and 30 b are reset. The variations ¹D² _(R), ¹D² _(L) in the registers 32 a, 32 b are added to the last values stored in the registers 27 a, 27 b by the adders 53 a, 53 b, and the results of the addition are held in the registers 27 a, 27 b. Accordingly, the contents of the registers 27 a, 27 b become below-described ¹N² _(R), ¹N² _(L) respectively.

¹ N ² _(R)=¹N¹ _(R)+¹ D ² _(R)  (A6)

¹ N ² L= ¹ N ¹ _(L)+¹ D ² _(L)  (A7)

[0065] The coordinate-determination part 23 determines the position of the point Q₂ as its coordinates Q₂ (x₂, Y₂) as in the case of determining the coordinates of the position of Q₁ from the three values of ¹N² _(R), ¹ N² _(L) and N_(K). The determined position is stored in the memory (not shown) or transferred to the external unit (not shown) such as a computer. In this embodiment, the frequency f_(S) of the ultrasonic signal is about 50 KHz and the value of h is preset to 1,000 so that the wave group of the ultrasonic signal is transmitted at intervals of about 20 ms. Under such conditions, if it is found, when F_(D)=3 MHz, that ¹Nc¹ _(R)=59, ¹Ns¹ _(R)=59,020, ¹Nc¹ _(L)=59, ¹Ns¹ _(L)=58,990, it is determined that ¹D² _(R)=20, ¹D² _(L)=−10, and subsequently it is determined that ¹N² _(R)=1,020, ¹N² _(L)=1,690. Accordingly, the coordinates of the position of the point Q₂ can be determined as Q₂ (618, 812) from the above-described three values including N_(K) (=2,100). What is meant by the above determination will be explained below. The pen is at the point Q₁ when the first wave group of the ultrasonic signal is emitted from the pen. The second wave group is emitted after a lapse of about 20 ms, which is equal to h·¹Nc¹ _(R)=1000×59=59,000 in the number of the clock pulses of the frequency F_(D), from the transmission of the first wave group, and the pen moves to the point Q₂ at that time. On the other hand, ¹Ns¹ _(R) and ¹Ns¹ _(L) represent, in the number of the clock pulses of the frequency F_(D), the times elapsed between the reception of the first wave group of the ultrasonic signal and the reception of the second wave group by the ultrasonic receivers. They vary depending on the distances between the point Q₁ and the ultrasonic receivers and the distances between the point Q₂ and the ultrasonic receivers. They decrease as the pen approaches the ultrasonic receivers and increase as the pen moves away therefrom. The variations ¹D² _(R), ¹D² _(L) represent variations of the distances between the pen and the ultrasonic receivers when the pen moves from the point Q₁ to the point Q₂. The determination of ¹D² _(R)=20 and ¹D² _(L) ⁼⁻¹⁰ indicates that the pen has moved away from the ultrasonic receiver 13 a and approached the ultrasonic receiver 13 b.

[0066] When a third wave group arrives, the counter circuits 28 a, 28 b start counting again, and the counter circuits 30 a, 30 b are reset to start counting in order to calculate ¹D³ _(R) and ¹D³ _(L) by the following equations as in the case of calculating the coordinates of the point Q₂.

¹ D ³ _(R)=¹ Ns ² _(R) −h· ¹ Nc ² _(R)  (A8)

¹ D ³ L= ¹ Ns ² _(L) −h· ¹ Nc ² _(L)  (A9)

[0067] Next, ¹N³ _(R) and ¹N³ _(L) are calculated by the following equations.

¹ N ³ _(R)=¹ N ² _(R)+¹ D ³ _(R)  (A10)

¹ N ³ _(L)=¹ N ² _(L)+¹ D ³ _(L)  (A11)

[0068] Accordingly, a new position Q₃ of the pen is determined as its coordinates Q₃ (x₃, y₃) by the values of ¹N³ _(R), ¹N³ _(L) and N_(K). Likewise, each time the wave group ¹ _(G) ^(j) arrives, ¹D^(j) _(R) and ¹D^(J) _(L) are calculated by the following equations.

¹ D ^(j) _(R)=¹ Ns ^((j−1)) _(R) −h· ¹ Nc ^((j−1)) _(R)  (A12)

¹ D ^(j) _(L)=¹ Ns ^((j−1)) _(L) −h· ¹ Nc ^((j−1)) _(L)  (A13)

[0069] Subsequently, ¹N^(j) _(R) and ¹N_(jL) are calculated by the following equations.

¹ N ^(j) _(R)=¹ N ^((j−1)) _(R)+¹ D ^(j) _(R)  (A14)

¹ N ^(j) _(L)+¹ N ^((j−1)) _(L)+¹ D ^(j) _(L)  (A15)

[0070] Thus, the coordinates Q_(j) (x_(j),y_(j)) of the point Q_(j) are successively determined, and stored in the not illustrated memory, or transferred to the external unit such as a computer (not shown).

[0071] When a second start signal ²S resulting from a second wave group ²e of the light signal is generated, all the counter circuits and all the registers except the fixed register 21 are reset. At that time, a wave group ¹g^((j+1)) of the ultrasonic signal is being emitted. This wave group ¹g^((j+1)) can be expressed as ²g¹. The second start signal undergoes the same process as the first start signal.

[0072] Thereafter, the above-described process is repeated until the pen is separate from the input plane and the not-illustrated switch of the pen is turned off. Then, the drive circuit 5 stops operating, and all the counter circuits and all the registers except the fixed register 21 are reset. Subsequently, a series of coordinates determined while the writing member 2 is in contact with the input plane is stored as data of one pen stroke in a memory or transferred to an external processing apparatus (not shown).

[0073] When the writing member 2 comes into contact with the input plane again and the switch of the input pen is turned on, the above-described process is performed to determine a series of coordinates of a second pen stroke and transferred to the external processing apparatus. The same process is repeated for each pen stroke.

[0074] As described above, in the first embodiment, a time period (pulse period) between receiving a pulse in a pulse train of the ultrasonic signal and receiving the succeeding pulse is clocked, and this clocked time period is multiplied by the ratio (a predetermined value: h) of the period of the ultrasonic pulse train to the pulse period to determined the time elapsed between emission of a pulse train of the ultrasonic signal and emission of the succeeding pulse train at the ultrasonic oscillator 3.

[0075] In the first embodiment, the period of emission of the light signal is longer than the period of emission of the ultrasonic signal. The ratio j of the period of emission of the light signal to the period of emission of the ultrasonic signal is 2 or more. When the light signal and the ultrasonic signal are emitted at the same time, the distances between the pen and the ultrasonic receivers are determined on the basis of timings of receptions of these signals. On the other hand, when only the ultrasonic signal is emitted, distance variations are calculated and added to the last-determined distances to determine the current distances. Such a distance-update by calculation of distance variations brings about a buildup of an error, and therefore it is necessary to emit the light signal at times (at intervals longer than the ultrasonic signal emission period) to determine the distances directly. With such a process, it is possible to reduce the drain of the battery built in the pen since the frequency of the light signal emission is low.

[0076] Furthermore, by emitting the ultrasonic signal at short intervals, it is possible to follow rapid movement of the pen and thereby provide data representing a smooth locus of the pen.

[0077] The drain of the battery decrease as the ratio j increases. When the ratio j is infinite, that is, if the light signal is emitted only once at the start of a pen-down state, electric power consumption can be minimum.

[0078] The second embodiment of the invention will now be described. The structure of the second embodiment is about the same as the first embodiment. FIG. 6 is a view explaining the difference between the first embodiment and the second embodiment. As shown in FIG. 6, a light-receiving circuit 34 is used instead of the light-receiving circuit 24 shown in FIG. 4, a counter circuit 35 is used instead of the counter circuits 28 a, 28 b shown in FIG. 4, and a register 35 is used instead of the registers 29 a, 29 b shown in FIG. 4.

[0079] As described above, in the first embodiment, the periods ^(n)Tc^(m) _(R), ^(n)Tc^(m) _(L) of the ultrasonic pulses of the frequency f_(S) and the time elapsed between emission of an ultrasonic pulse train and emission of the succeeding ultrasonic pulse train at the ultrasonic oscillator 3 are determined from the pulses of the ultrasonic reception signals ^(n)G^(m) _(R), ^(n)G^(m) _(L). On the other hand, the second embodiment is arranged such that the light signal E_(S) is processed by the light-receiving circuit 34 in FIG. 6 to generate a start signal (FIG. 5(E)) and a wave group ^(n)Ec (FIG. 5(F)) corresponding to the light signal E_(S), in order to determine the period ^(n)Tc of the ultrasonic pulses of the frequency f_(S) from the wave group ^(n)Ec. That is, in the second embodiment, the period ^(n)Tc of the pulses having the frequency f_(S) of the light signal E_(S) (the time elapsed between receiving one pulse of the light signal and receiving the succeeding pulse) is measured by the counter circuit 35 as the number ^(n)Nc of the clock pulses having the frequency F_(D) generated by the clock pulse generating circuit 22, and is stored in the register 36. The output ^(n)Nc of the register 36 is used as a substitute for ^(n)Nc^(m) _(R) and ^(n)Nc^(m) _(L) in the first embodiment. The output of the register 36 is multiplied by the factor h at the multipliers 51 a, 51 b to determine the time elapsed between emission of a ultrasonic pulse train and emission of the succeeding ultrasonic pulse train (the period of emission of the ultrasonic pulse train).

[0080] Thereafter, the other processes that are the same as those in the first embodiment are carried out to determine the coordinates.

[0081] As described above, in the second embodiment, the time elapsed between receiving an ultrasonic pulse train and receiving the succeeding ultrasonic pulse train is determined by multiplying the time between receiving a pulse of the light signal and receiving the succeeding pulse of the light signal by the factor h.

[0082] Although reference has been made to the cases where p=2 and q=2, it is obvious that the first embodiment allows p≧2 and q≧1, and the second embodiment allows p≧1 and q≧2. The factor h is preset such that h/f_(S), which is a coordinate-sampling interval, becomes a desired value that should be larger than the maximum propagation time required for the ultrasonic signal travels from the pen on the input plane to the ultrasonic receivers.

[0083] The third embodiment will now be described. In this embodiment, ^(n)Nc^(m) _(R) and ^(n)Nc^(m) _(L) measured in the first embodiment and ^(n)Nc measured in the second embodiment are not used, but [h/f_(S)]·F_(D) calculated beforehand is used instead of h·^(n)Nc^(m) _(R) and h·^(n)Nc^(m) _(L), or h·^(nNc). The third embodiment is a modification of the first embodiment shown in FIG. 4, in which the counter circuits 28 a, 28 b, the registers 29 a, 29 b, and the multipliers 51 a, 51 b are removed, and the output of a fixed register 40 storing the value of [h/f_(S)]·F_(D) is supplied to the subtracters 52 a, 52 b as shown in FIG. 7. It is obvious that the third embodiment allows p≧1 and q≧1. The third embodiment can be used when the frequency of emission of the ultrasonic wave group (f_(S)/h) is constant. If the frequency varies by a large amount, its position-detection accuracy is deteriorated. If the frequency variation is small, it is possible to reduce power consumption in the pen and to simplify the circuit structure of its receiver side without deteriorating the accuracy.

[0084] In the above-described first to third embodiments, the ultrasonic signal is emitted intermittently, whereas, in the below-described fourth to seventh embodiments, the ultrasonic signal is emitted continuously.

[0085]FIG. 8 shows a circuit structure of the fourth embodiment. The structure of the fourth embodiment is about the same as the first embodiment. The reference numerals in FIG. 8 identical to those in FIG. 4 represent the same or corresponding elements.

[0086] In this embodiment, the drive circuit 5 is replaced by a drive circuit 105 that causes the ultrasonic oscillator 3 to emit the ultrasonic signal having the frequency f_(S) continuously during the pen-down state. On the other hand, the drive circuit 105 causes the LED 4 to emit the light signal having a single pulse only once at the start of the pen-down state. This light signal serves as a start signal E_(R).

[0087] The counter circuits 126 a, 126 b receive the outputs of the clock pulse generating circuit 22, the light-receiving circuit 24 and the ultrasonic-receiving circuits 25 a, 25 b, and count the clock pulses from the moment of generation of the start signal E_(R) and the moment of generation of ultrasonic-reception signals. Thus, the times elapsed between the moment at which the ultrasonic signal is emitted from the ultrasonic oscillator 3 along with the start signal E_(R) and the moment at which it is received by the ultrasonic receivers 13 a, 13 b can be measured.

[0088] The registers 127 a, 127 b hold the counts of the counter circuits 126 a, 126 b after completion of the counts (time measurement).

[0089] The counter circuits 128 a, 128 b receive the outputs of the ultrasonic-receiving circuits 25 a, 25 b, and count the pulses of the ultrasonic-reception signals respectively. The counter circuits 129 a, 129 b receive the outputs of the clock pulse generating circuit 22 and the ultrasonic-receiving circuits 125 a, 125 b, and count the clock pulses respectively. As described later, the outputs of the counter circuits 128 a, 128 b and the counter circuits 129 a, 129 b, which are the count values for the same time period, are used to calculate distance variations in variation-calculating circuits 131 a, 131 b. The counts of the counter circuits 128 a, 128 b for a certain time period represent a time interval between emission of a certain part of the ultrasonic signal (m-th pulse, for example) and emission of a later part of this signal ((m+n) th pulse, for example) at the ultrasonic oscillator 3. The counts of the counter circuits 129 a, 129 b for the same time period represent the times elapsed until the ultrasonic receivers 13 a, 13 b receive the (m+n) th pulse after receiving the m-th pulse respectively. However, since the counter circuits 129 a, 129 b count the clock pulses of the frequency F_(D), while the counter circuits 128 a, 128 b count the pulses of the frequency f_(S), the registers 130 a, 130 b hold the counts of the counter circuits 129 a, 129 b after completion of the time measurement. Thereafter, the counter circuits 129 a, 129 b are reset (their counts are reset to the initial value 0) to provide for the next count.

[0090] The variation-calculating circuits 131 a, 131 b have multipliers 134 a, 134 b and subtracters 135 a, 135 b.

[0091] The multipliers 134 a, 134 b multiply the outputs of the counter circuits 128 a, 1278 b by the constant F_(D)/f_(S), respectively. It is for taking account of the frequency difference between the pulses counted by the counter circuits 128 a, 128 b and the pulses counted by the counter circuits 129 a, 129 b. Thus, the outputs of the multipliers 134 a, 134 b represent the time interval between emissions at the ultrasonic oscillator 3 in the same unit in which the elapsed time between receptions at the ultrasonic receivers 13 a, 13 b is represented.

[0092] The subtracters 135 a, 135 b subtract the outputs of the registers 130 a, 130 b from the outputs of the multipliers 134 a, 134 b, respectively. The outputs of the subtracters 135 a, 135 b, which are the outputs of the variation-calculating circuits 131 a, 131 b, represent variations of the distances from the input pen to the ultrasonic receivers 13 a, 13 b.

[0093] The registers 132 a, 132 b hold the outputs (distance variations) of the variation-calculating circuits 131 a, 131 b.

[0094] A variation-comparison circuit 133 determines whether the distance variations held in the registers 132 a, 132 b satisfy an expression (B6) or (B7) described later. If the variation-comparison circuit 133 determines that the expression (B6) or (B7) is satisfied, the values held in the registers 132 a, 132 b are added to the values held in the registers 127 a, 127 b. The results of the addition are held in the registers 127 a, 127 b.

[0095] The coordinate-determination part 23 calculates coordinates (for example, x-y coordinates) representing the position of the input pen by trigonometry from the value held in the fixed register 21, and the values representing the distances from the input pen to the ultrasonic receivers 13 a, 13 b held in the registers 127 a, 127 b. The result of the calculation is transferred to a not-illustrated memory or an external unit such as a computer, and held there.

[0096] The fourth embodiment will now be described with reference to FIG. 9 and FIG. 10. When the writing member 2 of the pen 1 comes into contact with the input plane 20 at the point Q₀ as shown in FIG. 9, the not-illustrated switch of the input pen is turned on to cause the ultrasonic oscillator 3 to emit the ultrasonic signal U_(S), and cause the LED 4 to emit the light signal ES at the same time through the drive circuit 105 shown in FIG. 8 (PEN-DOWN 1 in FIG. 10). As shown in FIG. 10(C), the ultrasonic signal U_(S) is oscillating at the frequency f_(S) while the writing member 2 is in contact with the input plane, that is, during the pen stroke. This ultrasonic signal U_(S) is received by the ultrasonic receivers 13 a, 13 b, and converted into ultrasonic-reception signals U_(R), U_(L) (electric pulse signals) by the ultrasonic-receiving circuits 25 a, 25 b. The light signal E_(S), which is a wave group including q pulses having the frequency p times (p being a positive integer) higher than the oscillation frequency f_(S) of the ultrasonic oscillator, is emitted only once at the start of each pen stroke. FIG. 10(A) shows a case where p=1 and q=1. The light signal E_(S) is received by the light-receiving element 14, and converted into a start signal E_(R) (electric pulse signal) indicating the start of the pen down state by the light-receiving circuit 24. The times ¹t_(R0), ¹t_(L0) between generation of the start signal E_(R) and generation of the ultrasonic-reception signals U_(R), U_(L) can be regarded as propagation times required for the ultrasonic signal to travel from the ultrasonic oscillator 3 to the ultrasonic receivers 13 a, 13 b, since the propagation time of the light signal is as small as negligible. The propagation times ¹t_(R0), ¹t_(L0) are clocked by the counter circuits 126 a, 126 b as the numbers N⁰ _(R), N⁰ _(L) of the clock pulses having the frequency F_(D)=3 MHz generated by the clock pulse generating circuit 22. The numbers N⁰ _(R), N⁰ _(L) of the clock pulses, which translate into the distances L⁰ _(R), L⁰ _(L) between the point Q₀ and the ultrasonic receivers 13 a, 13 b, are temporarily stored as equivalents of the distances in the registers 127 a, 27 b. The above equivalents N⁰ _(R), N⁰ _(L) are expressed in the same unit with N_(K) that is the equivalent of the distance between the ultrasonic receivers. As seen from FIG. 9, three line-segments connecting the above N⁰ _(R), N⁰ _(L) and N_(K) form a triangle. The coordinate-determination part 23 determines the position of the point Q₀ as its coordinates Q₀ (x₀,y₀) in the x-y coordinate system in accordance with the following equations (B2), (B3) by trigonometry. The coordinates determined are stored in a not-illustrated memory or transferred to an external unit such as a computer.

[N ⁰ _(R)]2=[x ₀]² +[y ₀]²  (B2)

[N ⁰ _(L)]2=[N _(K) −x ₀]² +[y ₀]²  (B3)

[0097] If N⁰ _(R)=1000 and N⁰ _(L)=1700 when F_(D)=3 MHz, x₀=600 and y₀=800 are calculated from N_(K)=2100, and are stored in a not illustrated memory or transferred to a not illustrated external unit such as a computer.

[0098] The switch of the pen remains closed while the input pen 1 is in contact with the input plane 20, that is, during the pen stroke. Accordingly, the oscillation of the ultrasonic signal U_(S) continues and the pulses of the ultrasonic reception signals U_(R), U_(L) are delivered continuously. The counter circuits 128 a, 128 b continue to count the numbers n_(R), n_(L) of these pulses, and the counter circuits 129 a, 129 b continue to count the numbers TR, TL of the clock pulses generated by the clock pulse generating circuit 22 at the frequency F_(D) as equivalents of the times t_(R), t_(L) elapsed from the starts of the ultrasonic reception signals U_(R), U_(L) respectively. The numbers TR, TL of the pulses in the counter circuits 129 a, 129 b when the counter circuits 128 a, 128 b count up to n¹ _(R), n¹ _(L) respectively, are successively stored in the registers 130 a, 130 b as TR¹ ₍n¹ _(R)) and TL¹ ₍n¹ _(L)). Furthermore, their respective variations DR¹ ₍n¹R₎, DL¹ ₍n¹ _(L)) are calculated every moment by the variation-calculating circuits 131 a, 131 b according to the following equations (B4) and (B5) and successively stored in the registers 132 a, 132 b respectively.

DR ¹ ₍ n ¹ _(R)) =TR ¹ ₍ n ¹ _(R)) −[n ¹ _(R)−1]·F _(D) / _(f) _(S)  (B4)

DL ¹ ₍ n ¹ L ₎ =TL ¹ ₍ n ¹ _(L)) −[n ¹ _(L)−1]·F _(D) /f _(S)  (B5)

[0099] The variation-comparison circuit 133 compares the absolute values of the variations DR¹ ₍n¹ _(R)), DL¹ ₍n¹ _(L)) with a predetermined value d to determine whether the following inequalities (B6), (B7) are satisfied.

|DR ¹ ₍ n ¹ _(R)) |≧d, |DL ¹ ₍ n ¹ _(L)) |<d (n¹ _(L)=1, 2, . . . n ¹ _(R))  (B6)

|DL ¹ ₍ n ¹ _(L)) |≧d, |DR ¹ ₍ n ¹ _(R)) |<d (n¹ _(R)=1, 2, . . . n ¹ _(L))  (B7)

[0100] If either of the inequalities (B6), (B7) is satisfied when an n¹-th pulse of the ultrasonic signal U_(S) emitted from the pen situated at the position Q₁, which had been moved from the Q₀, has arrived at the ultrasonic receivers, the variations DR¹ ₍n¹ ₎, DL¹ ₍n¹ ₎ corresponding to the n¹-th pulse are output from the registers 132 a, 132 b as the first variations DR¹, DL¹ and added to the values stored in the registers 127 a, 127 b by the adders 136 a, 136 b. The results of the addition are held in the registers 127 a, 127 b. Thus, the values stored in the registers 127 a, 127 b become distance-equivalent values N¹ _(R), N¹ _(L) expressed in the following equations (B8) and (B9).

N ¹ _(R) =N ⁰ _(R) +DR ¹  (B8)

N ¹ _(L) =N ⁰ _(L) +DL ¹  (B9)

[0101] The coordinate-determination part 23 determines the coordinates Q₁ (x₁,y₁) of the point Q₁ from the values of N¹ _(R), N¹ _(L) and N_(K), and stores them in the not illustrated memory or transfers them to the not illustrated external unit such as a computer.

[0102] The inequalities (B6), (B7) are satisfied when the distance variation exceeds the value of d. As described above, each time the distance variation exceeds the predetermined value, this variation is added to the distance value (the value stored in the register 127 a or 127 b) determined by the preceding calculation, and the result of the addition is stored in the register 127 a or 127 b as a new distance value. The coordinate-determination part 23 determines new coordinates based on this new distance value.

[0103] The calculation of the distance variation in the variation-calculating circuit 131 a or 131 b can be performed each time the number of the pulses counted by the counter circuit 128 a or 128 b reaches a predetermined number, or each time the number of the pulses counted by the counter circuit 129 a or 129 b reaches a predetermined number.

[0104]FIG. 10 shows a case where DR¹ ₁₁₁ becomes −18 when n¹ _(R)=111 and thereby the inequality (B6) (|DR¹ ₍n¹ _(R))|≧15) is first satisfied under the conditions of the oscillation frequency of the ultrasonic signal being 50 KHz and the value d being 15. Subsequently, when n¹ _(L)=101, DL¹ ₁₀₁ becomes 20 to satisfy the inequality (B7). However, the inequality (B6) is not satisfied since n¹ _(R) becomes 150 at that moment. Therefore, the value of DL¹ ₁₀₁ is added to the value in the register 127 b as DL¹=20, and n¹ _(L)=101=n¹. On the other hand, if the value of DR¹ ₁₀₁ stored in the register 132 a is −10 at that moment, the value of DR¹ ₁₀₁ is added to the value stored in the register 127 a as DR¹=−10. In consequence, the values of N¹ _(R) and N¹ _(L) held in the registers 127 a and 127 b are changed as N¹ _(R)=1010, N¹ _(L)=1680 respectively according to the equations (B8) and (B9). The coordinate-determination part 23 determines the coordinates of the point Q₁ as Q₁ (621, 797) from the three values of N¹ _(R)=1010, N¹ _(L)=1680 and N_(K)=2100. What is meant by the above determination will be explained below. When the ultrasonic oscillator 3 emits the first pulse, the writing member 2 of the pen 1 is at the point Q₀. The pen is moving, and therefore the writing member is at the point Q₁ when n¹ ₁=101, that is, 100·1/f_(S) after the emission of the first pulse. In other words, the writing member 2 is at the point Q₁ after 6000 (100·F_(D)/f_(S)=100×3000/50) clock pulses of the frequency F_(D) are generated when the 101st ultrasonic pulse is just emitted. At that moment, TR¹ ₁₀₁ and TL¹ ₁₀₁ represent, in the numbers of the clock pulses having the frequency F_(D), the times between reception of the first ultrasonic pulse and reception of the 101st ultrasonic pulse by the ultrasonic receivers 13 a, 13 b that are equivalents of the propagation times required for the ultrasonic signal travel from the point Q₁ to the ultrasonic receivers 13 a, 13 b. Accordingly, the variations DR¹ ₁₀₁, DL¹ ₁₀₁ represent a travel of the writing member 2 with respect to the ultrasonic receivers 13 a, 13 b corresponding to the movement of the writing member from the point Q₀ to the pint Q₁. What is meant by DR¹ ₁₀₁=−10 and DL¹ ₁₀₁=20 is that the pen is moving closer to the ultrasonic receiver 13 a and moving away from the ultrasonic receiver 13 b.

[0105] When either of the inequalities (B6), (B7) is satisfied, the counts of the counter circuits 128 a, 128 b are reduced by [n₁−1] to become n² _(R) and n² _(L) respectively to provide for the next counts. Furthermore, the count of the counter circuit 129 a is reduced by the value of TR¹ ₍n¹ ₁₎ held in the register 130 a to become TR² ₍n² _(R)), and the count of the counter circuit 129 b is reduced by the value of TL¹ ₍n¹ ₁₎ held in the register 130 b to become TL² ₍n² _(L)) to provide for the next counts. The register 130 a holding the values corresponding to TR¹ ₁ to TR¹ ₍n¹ ₎, and the register 130 b holding the values corresponding to TL¹ ₁ to TL¹ ₍n¹ ₎ are reset to provide for storing new values as TR² ₍n² _(R)) and TL² ₍n² _(L)) respectively. Likewise, the register 132 a holding the values corresponding to DR¹ ₁ to DR¹ ₍n¹ ₎, and the register 132 b holding the values corresponding to DL¹ ₁ to DL¹ ₍n¹ ₎ are reset to provide for storing new values as DR² ₍n² _(R)) and DL² ₍n² _(L)) respectively. A new variation DR² ₍n² _(R)) or DL² ₍n² _(L)) is calculated from TR² ₍n² _(R)) or TL² ₍n² _(L)) held in the register 130 a or 130 b which has been just reset and n² _(R) or n² _(L) on the basis of the following equation (B10) or (B11) which is equivalent of the equation (B4) or (B5). The register 132 a or 132 b holding data for more than n¹ pulses at the time of above reset is rewritten to have the above new variation to provide for the next operation.

DR ² ₍ n ² _(R)) =TR ² ₍ n ² _(R)) −[n ² _(R)) ]·F _(D) /f _(S)  (B10)

DL ² ₍ n ² _(R)) =TL2₍ n ² _(L)) −[n ² _(L)−1]·F _(D) /f _(S)  (B11)

[0106] In the case of FIG. 10 where the inequality (B7) is satisfied when n¹=101, the count of the counter circuit 128 a is 150, DL¹ ₁₀₁ held in the register 132 b is 20, and DR¹ ₁₀₁ held in the register 132 a is −10 when the count of the counter circuit 128 b reaches 101. At that moment, the variation-calculating circuits 131 a, 131 b output DR¹=−10 and DL¹=20, so that the count of the counter circuit 128 a is reduced by [101−1]=100 to change from 150 to n² _(R)=50, and the count of the counter circuit 128 b is reduced by 100 to change from 101 to n² _(L)=1. New counts start from this state. The count of the counter circuit 129 b is TL² ₁=0 since TL¹ ₁₀₁ (6000+20=6020)−TL¹ ₁₀₁=0, and the count of the counter circuit 129 a is, if TR¹ ₁₅₀ has reached 8925 at that moment, TR² ₅₀=2935 since TR₁₅₀(8925)−TR₁₀₁(6000−10=5990)=2935. New counts start from this state. The register 130 b holds TL²1=0, and the register 132 b holds DL²1=0 under such a reset state. On the other hand, the values corresponding to TR¹ ₁₀₁ to TR¹ ₁₅₀ held in the register 130 a at that moment are reduced by TR¹ ₁₀₁=5990 respectively and are left in the register 130 a as the values corresponding to TR² ₁ to TR² ₅₀. The register 132 a holding the values corresponding to DR¹ ₁₀₁ to DR¹ ₁₅₀ at that moment are rewritten to hold the values of DR² ₁ to DR² ₅₀ that are calculated from the values of TR² ₁ to TR² ₅₀ currently held in the register 130 a. For example, DR² ₅₀=TR² ₅₀−49×60=2935−2940=−5.

[0107] The moment at which the n¹-th pulse of the ultrasonic signal is emitted is used as a reference time for carrying out a second sampling. That is, a process similar to the above-described process for determining the coordinates of the point Q₁ is carried out regarding that a first pulse for the second sampling is emitted at this moment. If the pen is at the position Q₂ when an n²-th pulse of the ultrasonic signal is emitted after the new reference time, and the arrival of this n²-th pulse at the ultrasonic receiver satisfies one of the following inequalities (B12) and (B13) that are the equivalents of the inequalities (B6) and (B7),

|DR ² ₍ n ² _(R)) |≧d, |DL ² ₍ n ² _(L)) |<d (n² _(L)=1, 2, . . . n ² _(R))  (B12)

|DL ² ₍ n ² _(L)|) ≧d, |DR ² ₍ n ² _(R)|) <d (n² _(R)=1, 2, . . . n ² _(L))  (B13)

[0108] The variations DR² ₍n₂₎ and DL² ₍n² ₎ with respect to the n²-th pulse are output from the variation-calculating circuits 131 a, 131 b as variations DR², DL², and N² _(R) and N² _(L) are determined according to the following equations (B14) and (B15) that are the equivalents of the equations (B8) and (B9).

N ² _(R) =N ¹ _(R) +DR ²  (B14)

N² _(L) =N ¹ _(L) +DL ²  (B15)

[0109] The coordinate-determination part 23 determines the coordinates Q₂ (x₂,y₂) of the point Q₂ from the N² _(R), N² _(L) and N_(K), and stores them in the not illustrated memory or transfers them into the not illustrated external unit such as a computer.

[0110] By carrying out an m-th sampling, DR^(m) ₍n^(m) _(R)) and DL^(m) ₍n^(m) _(L)) are calculated according to the following equations (B16) and (B17).

DR ^(m) ₍ n ^(m) _(R)) =TR ^(m) ₍ n ^(m) _(R)) −[n ^(m) _(R)−1]·F _(D) /f _(S)  (B16)

DL ^(m) ₍ n ^(m) _(L)) =TL ^(m) ₍ n ^(m) _(L)) −[n ^(m) _(L)−1]·F _(D) /f _(S)  (B17)

[0111] Subsequently, when the following inequalities (B18) and (B19) are satisfied,

|DR ^(m) ₍ n ^(m) _(R)) |≧d, |DL ^(m) ₍ n ^(m) _(L)) |<d (n^(m) _(L)=1, 2, . . . n _(R))  (B18)

|DL ^(m) ₍ n ^(m) _(L)) |≧d, |DR ^(m) ₍ n ^(m) _(R)) |<d (n^(m) _(R)=1, 2, . . . n _(L))  (B19)

[0112] N^(m) _(R) and N^(m) _(L) are determined according to the following equations (B20) and (B21) as DR_(m) ₍n^(m) _(R))=D^(R) _(m), DL^(m) ₍n^(m) _(L))=DL^(m) _(L).

N ^(m) _(R) =N ^((m−1)) _(R) +DR ^(m)  (B20)

N ^(m) _(L) =N ^((m−1)) _(L) +DL ^(m)  (B21)

[0113] Then, the coordinates Q_(m) (x_(m),y_(m)) of the point Q_(m) are determined, and stored in the not illustrated memory or transferred to the not illustrated external unit such as a computer.

[0114] Thereafter, when the writing member 2 separates from the input plane 20, the switch (not illustrated) of the pen 1 is turned off, and thereby the oscillation of the ultrasonic oscillator 3 ceases (PEN-UP 1 in FIG. 10). In consequence, the counter circuits 126 a, 126 b, 128 a, 128 b, 129 a, and 129 b, and the registers 127 a, 127 b, 130 a, 130 b, 132 a, and 132 b are reset, and then a series of coordinates determined while the writing member 2 is in contact with the input plane 20 is stored in the memory or transferred to the external unit as stroke data.

[0115] When the writing member 2 again comes into contact with the input plane and the switch of the pen 1 is turned on, measurement of the propagation times ²t_(R0) and ²t_(L0) is started for determining the initial position of the pen in the second pen stroke (PEN-DOWN 2 in FIG. 10). By carrying out the above-described processes, a series of coordinates for the second pen stroke is determined and stored in the memory or transferred to the external unit.

[0116] In the above described embodiment, p=1 and q=1, however, it is obvious that p and q can be any positive integer.

[0117] The fourth embodiment described above can perform coordinate-determination smoothly even when the pen is moving fast, since the ultrasonic oscillator oscillates continuously during a pen stroke and timing of coordinate-sampling is determined based on the amount of the travel of the pen.

[0118] However, in the fourth embodiment, when determining coordinates of points after Q₁ in accordance with appropriate expressions such as equations (B16) and (B17), since [F_(D)/f_(S)] used in the multipliers 134 a, 134 b is a predetermined constant, an error may occur in the calculation due to pen-to-pen variation in the oscillation frequency f_(S). The fifth embodiment described below aims at solving this problem.

[0119] The fifth embodiment will now be described. The structure of the fifth embodiment is the same as the fourth embodiment as a whole. The difference is in the circuit structures of variation determination circuits 140 a, 140 b. The variation-calculating circuits 140 a, 140 b replacing the variation-calculating circuits 131 a, 131 b, have reference-time detecting circuits 141 a, 141 b, registers 142 a, 142 b, multipliers 143 a, 143 b, and subtracters 144 a, 144 b.

[0120] The reference-time detecting circuits 141 a, 141 b receive the output (clock pulses) of the clock pulse generating circuit 22 and the outputs of the ultrasonic-receiving circuits 25 a, 25 b to determine the ratio F_(D)/f_(S) by counting the clock pulses over one or more cycles of the ultrasonic signal. For example, if the count of the clock pulses for n cycles of the ultrasonic signal is m, F_(D)/f_(S) equals to m/n. To be more specific, in the fourth embodiment, the clock pulses generated by the clock pulse generating circuit 22 is counted over the period tc_(R) or tc_(L) of the first cycle of the ultrasonic reception signal U_(R) or U_(L) shown in FIG. 10(D) or (E) for each pen stroke, and the count N_(UR) or N_(UL) is determined as F_(D)/f_(S.)

[0121] The registers 142 a, 42 b hold the ratio F_(D)/f_(S) determined by the reference-time detecting circuits 141 a, 141 b.

[0122] The multipliers 143 a, 143 b multiply the outputs of the registers 142 a, 142 b by the outputs of the counter circuits 128 a, 128 b respectively.

[0123] The subtracters 144 a, 144 b subtract the outputs of the multipliers 143 a, 143 b from the outputs of the registers 130 a, 130 b respectively.

[0124] The multipliers 143 a, 143 b, and the subtracters 144 a, 144 b are used to calculate distance variations D′R^(m)(n^(m) _(R)) and D′L^(m)(n^(m) _(L)) in accordance with the following equations (B22), (B23).

D′R ^(m) ₍ n ^(m) _(R)) =TR ^(m) ₍ n ^(m) _(R)) −[n ^(m) _(R)−1]·N _(UR)  (B22)

D′L ^(m)(n ^(m) _(L))=TL ^(m)(n ^(m) _(L))−[n ^(m) _(L)−1]·N _(UL)  (B23)

[0125] The distance variations D′R^(m)(n^(m) _(R)) and D′L^(m)(n^(m) _(L)) are stored in the registers 132 a, 132 b. The other processes carried out in the fifth embodiment are the same as those carried out in the fourth embodiment.

[0126] As described above, the fifth embodiment differs from the fourth embodiment in that [F_(D)/f_(S)] is replaced by N_(UR) or N_(UL) in the equations (B16), (B17)

[0127] In the fifth embodiment, since F_(D)/f_(S) is determined by actually measuring the oscillation frequency of the ultrasonic oscillator of the pen by counting the clock pulses, it is possible to eliminate errors due to pen-to-pen variation in the oscillation frequency f_(S).

[0128] The sixth embodiment where p=3 and q=2 will be described below. The structure of the sixth embodiment is about the same as that of the fourth embodiment. The difference is in a different light-receiving circuit 150 and a variation-calculating circuit 151 shown in FIG. 12.

[0129] The light-receiving circuit 150 replacing the light-receiving circuit 24 in the fourth embodiment includes a pulse-generating circuit 152 and a start signal generator 153. The pulse-generating circuit 152 converts a light signal E_(P) (FIG. 13(B)) into a pulse signal as a light reception signal E_(C) (FIG. 13(C)).

[0130] The start signal generator 153 generates, in response to the output of the pulse-generating circuit 152, a start signal E_(R) which is an equivalent of the start signal E_(R) in the fourth embodiment.

[0131] The variation-calculating circuit 151, which replaces the variation-calculating circuits 140 a, 140 b, includes a reference-time detecting circuit 154, a register 155, multipliers 143 a, 143 b, and subtracters 144 a, 144 b.

[0132] The reference-time detecting circuit 154 measures a period t_(S) of a first cycle of the light-reception signal E_(C) by counting the number N_(S) of the clock pulses generated by the clock pulse generating circuit 22 at the frequency F_(D). The register 155 holds the result of the measurement.

[0133] The multipliers 143 a, 143 b, and the subtracters 144 a, 144 b are the same as those shown in FIG. 11. But one input of each of the multipliers 143 a, 143 b is connected to the register 155. FIG. 13(A) shows a waveform of the ultrasonic signal U_(S) that is the same as the one in the fourth embodiment.

[0134] The multipliers 143 a, 143 b, and the subtracters 144 a, 144 b are used to calculate distance variations D″R^(m) ₍n^(m) _(R)), and D″L^(m) ₍n^(m) _(L)) according to the following equations B(24), B(25).

D″R ^(m) ₍ n ^(m) _(R)) =TR ^(m) ₍ n ^(m) _(R)) −[n ^(m) _(R)−1]·p(=3)·N _(S)  (B24)

D″L ^(m) ₍ n ^(m) _(L)) =TL ^(m) ₍ n ^(m) _(L)) −[n ^(m) _(L)−1]·p(=3)·N _(S)  (B25)

[0135] These calculated variations D″R^(m) ₍n^(m) _(R)) and D″L^(m) ₍n^(m) _(L)) are stored in the registers 132 a, 132 b. The other processes are the same processes as those in the fourth embodiment. That is, [F_(D)/f_(S)] in the fourth embodiment is replaced by the above-described N_(S). It is needless to say that the processes for determining coordinates used in the fourth embodiment can be used in the sixth embodiment as well, and q may be greater than two.

[0136] As described above, the sixth embodiment is arranged to count the clock pulses for one period of the light signal and obtain the product of the value of this count and the ratio of the period of the light signal to that of the ultrasonic signal as an equivalent of [F_(D)/f_(S)].

[0137] The seventh embodiment of the invention where p=1 and q is infinite, that is, a case of continuous emission will now be described.

[0138] The structure of the seventh embodiment is about the same as that of the sixth embodiment. The difference is in a variation-calculating circuit shown in FIG. 14. FIG. 15 shows signal waveforms in the seventh embodiment.

[0139] This variation-calculating circuit 160 has a counter circuit 161, a register 162, and multipliers 143 a, 143 b.

[0140] The counter circuit 161 measures a time period between a rise of a first pulse and a rise of an n-th pulse of the light-reception signal E_(C) (FIG. 15(c)) corresponding to the light signal E_(P) (FIG. 15(B)) in an m-th (m=1,2,3, . . . ) sampling by counting the number of the clock pulses having the frequency FD as TV^(m) (n^(m)). The results of this measurement are successively stored in the register 162. The subtracters 144 a, 144 b subtract the output of the register 162 from the outputs of the registers 128 a, 128 b respectively. That is, the distance variations D′″R^(m) ₍n^(m) _(R)) and D′″L^(m) ₍n^(m) _(L)) are calculated according to the following equations (B26), (B27).

D′″R ^(m) ₍ n ^(m) _(R)) =TR ^(m) ₍ n ^(m) _(R)) −TV ^(m) ₍ n ^(m) ₎  (B26)

D′″L ^(m) ₍ n ^(m) _(L)) =TL ^(m) ₍ n ^(m) _(L)) −TV ^(m) ₍ n ^(m) ₎  (B27)

[0141] Thus caslculated D′″R^(m) ₍n^(m) _(R)) and D′″L^(m) ₍n^(m)L₎ are stored in the registers 132 a, 132 b. The other processes are the same as those in the fourth embodiment.

[0142] As described above, in the seventh embodiment, the light signal is emitted continuously in synchronization with the ultrasonic signal and the pulses of the light signal received by the light-receiving element are counted to measure an elapsed time between emission of the m-th pulse and emission of the (m+n)-th pulse of the ultrasonic signal at the ultrasonic oscillator.

[0143] The sixth and the seventh embodiments are arranged to measure the frequency of the light signal emitted from the pen side in synchronization with the ultrasonic signal at the receiving side, and to calculate the distance variation based on the measured frequency. Therefore, they can determine coordinates accurately without being affected by the pen-to-pen variation in the emitting frequency.

[0144] The eighth embodiment, which is a variant of the fourth to seventh embodiments, will now be described.

[0145] In the fourth to seventh embodiments, the timing of coordinate acquisition (sampling timing) after Q₁ is determined to be an instant at which the inequality (B18) or (B19) is satisfied. However, it is also possible to initiate a sampling each time the number of the counted pulses of the continuously oscillating ultrasonic signal from the pen reaches a predetermined value. Accordingly, in this embodiment, the successive calculations of variations in the fourth to seventh embodiments are not performed, but calculations of D′″R^(m) and D′″L^(m) are performed instead according to the following equations (B28), (B29).

D′″R ^(m) =TR ^(m) −J  (B28)

D′″L ^(m) =TL ^(m) −J  (B29)

[0146] where

[0147] C^(m) is a variable coefficient;

[0148] J=C^(m)·F_(D)/f_(S) . . . (when applied to the fourth embodiment);

[0149] J=C^(m)·N_(UX) (X:R or L) . . . (when applied to the fifth embodiment)

[0150] J=C^(m)·N_(S) . . . (when applied to the sixth embodiment); and

[0151] J=TV^(m) _(c) ^(m) ₎ . . . (when applied to the seventh embodiment).

[0152] Here, D′″R^(m) and D′″L^(m) are the same as DR^(m) and DL^(m) within the equations (B20) and (B21) used in the fourth to seventh embodiments, and Cm is set according to the followings. At the start of the stroke, that is, at the time of sampling Q₁, a fixed value C¹ is used. For subsequent samplings, C^(m) is determined by comparing the absolute values of the last calculated D′″R^(m) and D′″L^(m) with the predetermined values d_(L) and d_(S) (d_(L)>d_(S)). To be more specific, C^(m) is determined from an arithmetic progression C_(n) prepared beforehand to satisfy the followings.

When |D′″R ^(m) |≧d _(L) or |D′″L ^(m) |≧d _(L)  (B30),

C ^((m+1)) <C ^(m)  (B31).

When |D′″R ^(m) |≦d _(S) or |D′″L ^(m) |≦d _(S)  (B32),

C ^((m+1)) >C ^(m)  (B33).

[0153] When the inequalities (B30) and (B32) are not satisfied,

C ^((m+1)) =C ^(m)  (B34).

[0154] The ninth embodiment, which is a variant of the fourth to eighth embodiments, will now be described. The fourth to eighth embodiments are described as “2-ultrasonic-wave+electromagnetic-wave type” system using a pen having an ultrasonic oscillator and a light emitter, however, they can be a “3-ultrasonic-wave type” system.

[0155] In this case, at least three ultrasonic receivers X, Y, Z are disposed on the same plane to receive the ultrasonic signal from the pen as shown in FIG. 16. When the pen comes into contact with the plane at the point G₀, the ultrasonic oscillator of the pen starts to emit the ultrasonic signal continuously. Then a hyperbolic curve w1 in the x′-y′ coordinate system including the above X, Y, Z is determined from the difference of timings of receptions between the receivers X and Y, and likewise, another hyperbolic curve w2 in the x′-y′ coordinate system is determined from the difference of timings of receptions between the receivers Y and Z. The position of the point G₀ is determined as the point of intersection of these hyperbolic curves w1 and w2. Subsequently, the distances I_(x) and I_(y) between the point G₀ thus determined and the receivers X, Z are determined. Thereafter the same processes as those in the fourth, fifth or eighth embodiment is performed for determining coordinates of the position G_(m) of the pen.

[0156] The tenth embodiment where the distance between the receivers 13 a and 13 b is made variable to adapt to input mediums of various widths will now be described.

[0157] The structure of this embodiment is about the same as the first embodiment. The difference is in that the ultrasonic receiver 13 b is mounted so at to be position-adjustable as shown in FIG. 17, and the signal processor 15 is modified accordingly as shown in FIG. 18.

[0158] The signal processor 15 includes a coordinate-determination circuit 223 corresponding to the coordinate-determination part 23 of FIG. 8, a light-receiving circuit 224 corresponding to the light-receiving circuit 24 of FIG. 8, ultrasonic-receiving circuits 225 a, 225 b corresponding to the ultrasonic-receiving circuits 25 a, 25 b of FIG. 8, counter circuits 226 a, 226 b corresponding to the counter circuits 126 a, 126 b of FIG. 8. The signal processor 15 further includes a distance-measuring device 220 for measuring a variable distance K′ between the ultrasonic receivers 13 a and 13 b, a coordinate-conversion circuit 222, and a data storage 228 for storing coordinates after coordinate-conversion.

[0159] At the time of writing into the form 20, the position of the receiver 13 b is adjusted such that the distance K′ is about the same as the length of the side AB of the form which is in contact with or close to the main unit 11. The distance K′ measured by the distance-measuring device 220 is input to the coordinate-determination circuit 223 to be used as an equivalent of N_(K) representing the distance between the receivers 13 a and 13 b. Then the points C, D at each end portion of the sides AB are pressed in turn by the pen to acquire x-y coordinates of the points C, D. Then a new x′-y′ coordinate system whose origin point is the point C and whose x′ axis is a straight line passing through the points C, D is established. After the process for sampling and acquiring x-y coordinates as described in the first embodiment is performed, the coordinate-conversion circuit 222 converts the x-y coordinates into x′-y′ coordinates and stores them in the data storage 228.

[0160] With the tenth embodiment, it is possible to handle forms of various sizes. Especially, when a form of small size is used, since resolution in position determination is increased, small characters can be recognized accurately.

[0161] Although reference has been made to the cases where a light signal is emitted along with an ultrasonic signal from the pen as described in the first to eighth embodiments, electromagnetic waves for wireless transmission can be used instead of the light signal. The present invention is also applicable to a case where a timing signal is transmitted through a cable from the pen to the signal processor of the main unit. The coordinate-capturing apparatus of the present invention is not limited to the above described ultrasonic aerial-propagation type. It may be an ultrasonic in-medium propagation (surface acoustic wave) type. Furthermore, although it has been described that the timing signal and the ultrasonic signal are emitted only during the pen-down state, the present invention is applicable to a case where they are emitted all the time.

[0162] The above explained preferred embodiments are exemplary of the invention of the present application which is described solely by the claims appended below. It should be understood that modifications of the preferred embodiments may be made as would occur to one of skilled in the art. 

1. A coordinate-capturing apparatus comprising: an ultrasonic transmitter mounted on an input pen to be in contact with a medium for transmitting ultrasonic waves; a first and a second ultrasonic receivers for receiving ultrasonic waves transmitted form said ultrasonic transmitter; a distance calculating device for calculating distances between said input pen and said first and said second ultrasonic receivers when a specific condition is satisfied; a coordinate calculating device for calculating coordinates representing a position of said input pen on the basis of said distances calculated by said distance calculating device; and a distance variation calculating device for calculating variations of said distances between said input pen and said first and second ultrasonic receivers on the basis of time periods between receptions of an ultrasonic wave transmitted from said ultrasonic transmitter at a first time by said first and second ultrasonic receivers respectively and receptions of an ultrasonic wave transmitted from said ultrasonic transmitter at a second time by said first and second ultrasonic receivers respectively; said distance calculating device calculating, on the basis of said distances calculated when said specific condition is satisfied and said variations of said distances calculated by said distance variation calculating device, distances between said input pen and said first and second ultrasonic receivers when said specific condition is not satisfied.
 2. A coordinate-capturing apparatus according to claim 1 , in which: said ultrasonic transmitter transmits ultrasonic waves intermittently at regular intervals; said coordinate-capturing apparatus further includes a clock generator for generating clock pulses continuously; and said distance variation calculating device includes: a device for determining first time periods between said first time and said second time for said first and second ultrasonic receivers; a device for determining second time periods between third times at which ultrasonic waves transmitted from said ultrasonic transmitter at said first time is received by said first and second ultrasonic receivers respectively and fourth times at which ultrasonic waves transmitted from said ultrasonic transmitter at said second time is received by said first and second ultrasonic receivers respectively; and a device for detecting difference between said first time periods and second time periods to determine said distance variations.
 3. A coordinate-capturing apparatus according to claim 2 , in which: said ultrasonic transmitter transmits intermittently ultrasonic waves having a first period at a second period; said device for determining said first time periods includes a device for counting said clock pulses generated during said first period of ultrasonic waves received by said first and second ultrasonic receivers to obtain first count values for said first and second ultrasonic receivers and a device for obtaining, by multiplying said first count values by a ratio of said second period to said first period, count values which are equivalent of said first time periods; said device for determining said second time periods includes a device for counting said clock pulses generated between said third times and said fourth times respectively to obtain second count values; and said distance variations are determined on the basis of difference between said first count values and said second count values.
 4. A coordinate-capturing apparatus according to claim 2 , in which: said input pen includes a timing signal transmitter for transmitting a timing signal of a train of timing pulses having a first period; said coordinate-capturing apparatus further includes a timing signal receiver for receiving said timing signal; said ultrasonic transmitter transmits ultrasonic pulses at a second period; said device for determining said first time periods includes a device for counting said clock pulses generated during said first period of said timing signal received by said timing signal receiver to obtain a first count value, and a device for obtaining count values which are equivalent of said first time periods by multiplying said first count value by a ratio of said second period to said first period; said device for determining said second time periods includes a device for counting said clock pulses generated between said third times and fourth times to obtain second count values; and said distance variations are determined on the basis of difference between said first count values and said second count values.
 5. A coordinate-capturing apparatus according to claim 2 , in which said device for determining said first time periods includes a device for holding a predetermined value corresponding to the number of clock pulses to be generated between said first and second times.
 6. A coordinate-capturing apparatus according to claim 1 , in which: said ultrasonic transmitter transmits ultrasonic waves continuously while said input pen is in contact with said medium; said distance variation calculating device calculates said distance variations periodically, and when said distance variations exceed a predetermined value for any of said first and second ultrasonic receivers, said distance calculating device updates said distances between said input pen and said ultrasonic receivers, and said coordinate calculating device calculates new coordinates.
 7. A coordinate-capturing apparatus according to claim 6 , in which said coordinate-capturing apparatus further includes a clock generator for generating clock pulses continuously, and said distance variation calculating device includes: a device for determining first time periods between first times and second times for said first and second ultrasonic receivers; a device for determining second time periods between third times at which ultrasonic waves transmitted from said ultrasonic transmitter at said first times is received by said first and second ultrasonic receivers respectively and fourth times at which ultrasonic waves transmitted from said ultrasonic transmitter at said second times is received by said first and second ultrasonic receivers respectively; and a device for obtaining differences between said first and second time periods to determine said distance variations.
 8. A coordinate-capturing apparatus according to claim 7 , in which said device for determining said first time periods includes first counters for counting received ultrasonic waves up to a first value, and said device for determining said second time periods includes second counters for counting said clock pulses until a count of corresponding one of said first counters reaches said first value.
 9. A coordinate-capturing apparatus according to claim 7 , in which said device for determining said first time periods determines said first time periods by obtaining products of the numbers of ultrasonic waves received by said first and second ultrasonic receivers respectively and ratios of a frequency of said clock pulses to frequencies of ultrasonic waves received by said first and second ultrasonic receivers respectively.
 10. A coordinate-capturing apparatus according to claim 9 , in which said device for determining said first time periods includes a device for performing multiplication using a predetermined value as said ratios of a frequency of said clock pulses to said frequencies of ultrasonic waves.
 11. A coordinate-capturing apparatus according to claim 9 , in which said device for determining said first time periods includes a device for measuring actual frequencies of ultrasonic waves received by said first and second ultrasonic receivers, to obtain said ratios on the basis of said actual frequencies.
 12. A coordinate-capturing apparatus according to claim 11 , in which said device obtaining said ratios obtains said ratios by counting said clock pulses generated during one period of ultrasonic waves received by said first and second ultrasonic receivers respectively.
 13. A coordinate-capturing apparatus according to claim 11 , in which said input pen has a timing signal transmitter for transmitting a timing signal of a pulse train, and said device determining said ratios obtains said ratios by counting said clock pulses generated during one period of pulses of said timing signal, and by obtaining products of the number of said clock pulses counted during said one period and ratios of a period of said timing signal to periods of ultrasonic waves received by said first and second ultrasonic receivers respectively.
 14. A coordinate-capturing apparatus according to claim 9 , in which said input pen has a timing signal transmitter for continuously transmitting a timing signal of a pulse train in synchronization with ultrasonic waves, and said device for determining said first time periods counts pulses of said timing signal received by a timing signal receiver to determine said first time periods.
 15. A coordinate-capturing apparatus according to claim 1 , in which said coordinate calculating device calculates coordinates representing a position of said input pen on the basis of a distance between said first and second ultrasonic receivers and distances between said input pen and said first and second ultrasonic receivers.
 16. A coordinate-capturing apparatus according to claim 1 , in which said specific condition includes a contact between said input pen and said medium.
 17. A coordinate-capturing apparatus according to claim 16 , in which said specific condition further includes a transmission of a timing signal at regular time intervals after contact between said input pen and said medium.
 18. A coordinate-capturing apparatus according to claim 1 , in which said input pen further includes a timing signal transmitter, said coordinate-capturing apparatus further includes a timing signal receiver, and said distance calculating device calculates distances between said input pen and said first and second ultrasonic receivers on the basis of times elapsed between transmission of ultrasonic pulses from said ultrasonic transmitter and receptions of said ultrasonic pulses by said first and second ultrasonic receivers.
 19. A coordinate-capturing apparatus according to claim 1 , further including a third ultrasonic receiver for receiving ultrasonic waves transmitted from said ultrasonic transmitter, said distance calculating device calculating distances between said input pen and said first and second ultrasonic receivers on the basis of ultrasonic waves received by said first, second, and third ultrasonic receivers when said specific condition is satisfied.
 20. A coordinate-capturing apparatus according to claim 1 , in which a distance between said first and second ultrasonic receivers is adjustable. 